A typical structure of a magnetic recording medium (magnetic disc) for use with a fixed magnetic disc drive is shown in FIG. 1 by way of example. To produce the recording medium, a non-magnetic substrate 11 formed of Al--Mg alloy is plated with Ni--P by electroless plating or deposition, to form a non-magnetic metal layer 12, so as to provide a non-magnetic base 1 consisting of the non-magnetic substrate 11 and the non-magnetic metal layer 12. After a non-magnetic metal base layer 2 is laminated on the non-magnetic base 1, a magnetic layer 3 in the form of a thin film made of a ferromagnetic alloy, such as Co--Cr--Ta or Co--Cr--Pt, is laminated on the metal base layer 2, and a carbon protective layer 4 is formed on the magnetic layer 3. The carbon protective layer 4 is then coated with a liquid lubricant which provides a lubricant layer 5. In this manner, a magnetic disc as shown in FIG. 1 is formed.
The non-magnetic base 1 may be selected from an alumite base, a glass base, a ceramic base, and others. The base 1 is formed with minute protrusions and recesses by texturing after it is polished, and the non-magnetic metal base layer 2 made of Cr is formed on the base 1 by sputtering in an Ar atmosphere while the base 1 is being heated to about 200.degree. C. In addition, the magnetic layer 3, and the carbon protective layer 4 made of amorphous carbon are successively formed on the non-magnetic metal base layer 2. The carbon protective layer 4 is then coated with a perfluoropolyether lubricant, to thus provide the magnetic recording medium.
The magnetic recording medium or disc fabricated in the above manner is installed in a fixed magnetic disc drive. When the magnetic disc is rotated in the operation of the magnetic disc drive, the disc is repeatedly brought into contact with a recording head of the disc drive while being rotated at a constant speed. Namely, a CSS (contact start/stop) system is employed wherein the magnetic disc drive stops rotating the magnetic disc when the recoding head and the magnetic disc surface come into contact with each other, and when the disc drive starts operating, the recording head floats or flies slightly above the surface of the magnetic disc, so as to read and write information. In this system, the recording head is held in contact with the magnetic disc surface for most of the time, and is brought into a non-contact state (where the recording head slightly floats above the magnetic disc) only when the disc drive is operating. Due to the sliding movement of the recording head relative to the magnetic disc, friction occurs between the head and the surface of the magnetic disc. In order to protect the magnetic layer 3 from such friction and other problems, the carbon protective layer 4 and the lubricant layer 5 are formed on the magnetic layer 3.
In the magnetic disc as described above, the protective layer is generally made of carbon, which is often formed into a film by sputtering or CVD in an Ar atmosphere. One of the reasons why carbon is used as a material for forming the protective layer is that an amorphous carbon layer formed by sputtering has relatively strong graphitic properties, and therefore shows a relatively low coefficient of friction in the atmosphere containing water, which is a typical property of graphite.
The carbon protective layer, however, is likely to wear due to its relatively low hardness as compared with a ceramic material, such as Al.sub.2 O.sub.3.TiC or CaTiO.sub.3, which is used for forming a slider of a thin-film head or MIG head, and the wear of the protective layer may result in head crush in some cases. To solve this problem, studies have been made in an attempt to develop protective layer having sufficiently high hardness. In recent years, there have been widely used protective layers formed of diamond-like carbon (DLC) having properties similar to those of diamond having extremely high hardness, or those formed by adding a small amount of N or Si to amorphous carbon or diamond-like carbon. In the diamond-like carbon protective layer, the proportion of carbon atoms bonded in a diamond structure is higher than that of carbon atoms bonded in a graphite structure.
The lubricant layer that provides the top layer of the magnetic disc needs to be stably formed with uniform thickness on the surface of the protective layer. It is also important that the lubricant layer exhibit high adhesiveness and bonding strength with the protective layer. To increase the adhesiveness, a lubricant layer has been proposed which is composed of perfluoropolyether having various types of polar group at a molecular end or ends.
The perfluoropolyether lubricant has a poor lubricating characteristic if its molecular weight is too low, and tends to adhere to the recording head if the molecular weight is too high. Thus, perfluoropolyether lubricants having the weight average molecular weight (Mw) of 1500 to 5500 have been conventionally used.
The perfluoropolyether lubricants which have been conventionally used include those containing an aromatic ring or its derivative as a polar group at a molecular end or ends (for example, FOMBLIN AM2001 available from Ausimont S.p.A., and DEMNUM-SP available from Daikin Industries, Ltd.), those containing a hydroxyl group as an end polar group (for example, FOMBLIN Z-DOL or Z-Tetraol available from Ausimont S.p.A., and DEMNUM-SA available from Daikin Industries, Ltd.), and those containing a carboxyl group as an end polar group (for example, FOMBLIN Z-DIAC available from Ausimont S.p.A., DEMNUM-SH available from Daikin Industries, Ltd., and KRYTOX-FS available from DuPont, Japan).
With a rapidly increasing demand for high-density recording in recent years, the fly height of the magnetic head above the magnetic disc has been reduced, and a low-height fly head, such as Tri-pad head or Tri-omega head, has been increasingly employed in place of known TPC head. With the fly height thus reduced, a negative pressure is likely to be generated between the magnetic head and the magnetic disc when the magnetic head slides above the surface of the magnetic disc that is rotating at a high speed, with a result of transfer of the lubricant from the magnetic disc surface to the magnetic head. If the lubricant is transferred onto the magnetic head, the head is contaminated, and flying characteristics of the magnetic head are disturbed (for example, the fly height is increased), resulting in reduction of the reproduction output. If a large amount of the lubricant is transferred to the magnetic head, so-called fly-stiction (adhesion of the head) occurs when the magnetic head re-starts operating after it is stopped for a while.
As the recording density and speed increase, the rotating speed of the magnetic disc in the disc drive increases from a conventional speed, i.e., 3600 rpm, to a considerably high speed, i.e., 7200 to 10000 rpm. As a result, a phenomenon called "spin migration" is likely to appear, namely, the lubricant on the magnetic disc surface moves or dissipates toward a radially outer portion of the disc due to a centrifugal force. If the degree of migration becomes large, head crush may occur in a radially inner portion of the disc, or adhesion of the head to the magnetic disc (fly stiction) may occur in the radially outer portion of the disc.
In the meantime, magnetic disc drives used in these days have a completely enclosed structure in which the interior space of the disc drive is isolated or sealed from the exterior. If such a magnetic disc drive is used under a condition of high humidity, gas generated from internal components of the disc drive fills the interior space, and the concentration of the gas is increased. The gas component thus generated is dissolved into water produced due to high humidity or dew formation, thereby to produce a harmful acid gas, which eventually acts on the surface of the magnetic disc.
On the other hand, the surface of the carbon protective layer is covered with a thin oxide film having a functional group, such as reactive carbonyl group, carboxyl group, or hydroxyl group, and the above-indicated polar functional group at a chain end of the liquid lubricant is bonded to and reacts with the functional group of the oxide film. Under a high-humidity environment where water is present, however, the degree of interaction between these functional groups is reduced, and the harmful acid gas as described above actively adheres to portions of the disc surface where the bonding strength is reduced. Further, the material of the recording head is more likely to undergo a catalytic action or generate heat due to friction, through contact recording with reduced fly height of the magnetic head. The deposition of the acid gas on the disc surface, combined with the catalytic action or heat due to friction, accelerates decomposition of a main chain portion (ether portion) of the perfluoropolyether lubricant. Substances resulting from the decomposition, or corrosive components of the gas, or the like, deposited on the disc surface, are transferred to the surface of the magnetic head, and the fly characteristic of the magnetic head is disturbed or deteriorated, thus causing reduction in the reproduction output. In addition, the perfluoropolyether lubricant thus decomposed cannot maintain its lubricating characteristic, thereby causing wear of the protective film, and in the worst case, head crush takes place.
Although various attempts have been made to produce magnetic discs using perfluoropolyether lubricants with various types of polar functional group, so as to solve the above-described problems, such a magnetic disc that meets with all of the above demands had not been developed.